1. Field of the Invention
The present invention relates generally to a pixel sensor having an Avalanche photodiode, and more particularly, to a 2D/3D dual-mode image sensing circuitry.
2. Description of the Related Art
Among the existing three-dimension (3D) digital cameras, one is composed of two complementary metal oxide semiconductor (CMOS) sensors that can take photographs of one identical frame from two different angles at the same time, while it is intended to take a 3D photograph, and then synthesize a 3D stereoscopic image via an image processor built in the digital camera. How the human eyes can see the stereoscopic image of an object is based on that the light reflected by the object enters the retinas of two eyes, then the existence and shape of the object are perceived, and finally the stereoscopic perception of the object is generated due to the binocular parallax. Thus, the stereoscopic image taken by the camera can be displayed straight on the screen of the camera and the rays distributively enter two eyes in such a way that the binocular parallax results in the stereoscopic perception. The typical camera based on such technique is FUJI FinePix REAL 3D W1. However, such binocular parallax is defective because the viewpoint is fixed for the captured images. Once the viewpoint is changed, the stereopsis disappears.
Another 3D digital camera based on the time of flight is though also composed of two CMOS sensors, but one of them is to detect the gray-level value of an object and the other is to detect the depth information of the same object. The gray-level value and the depth information are integrated to synthesize the 3D stereoscopic image. Such technique comes up with the 2D and 3D information of the whole image, so that the viewpoint can be changeable and the synthesized 3D stereoscopic images are variable subject to different viewpoints. Although the individual 2D and 3D information can be processed synchronously, the two sensors detect the object at different positions, such that the binocular parallax needs to be complemented at the back end.
In the 3D camera, the CMOS sensor for sensing the object depth generally includes an Avalanche photodiode (APD), which is driven by close to its breakdown voltage with consideration of avoiding breakdown; the yield value can be deemed indefinite owing to a high current gain upon receipt of micro photons for detecting very weak light in a rapidly operating pixel circuit. As for the 2D camera, it focuses on dynamic range and resolution; the larger the dynamic range is, the more the maximum unsaturated signal can be detected and the larger the sensitivity range is in such a way that the captured color quality is closer to the ideal one.
So far, most of the patents in the prior art regarding the 3D image, sensor based on the time of flight for measuring the depth information improved the environment and method of measurement or the storage manners of image data. For example, U.S. Pat. No. 5,675,407 disclosed that the improvement was done by different wavelengths of colored lights under the time of flight. In other prior art, a phase meter was employed for detecting the depth information. The above prior art was usually based on the integration of the avalanche photodiode and the pixel circuit or improved the manufacturing process for rapid reaction. However, such prior art had a significant defective; namely, the 3D imaging can though come up with accurate depth information by a high-speed operation, but the dynamic range is not good. For example, in U.S. Pat. Nos. 7,547,872, 7,262,402 & 6,384,663, the avalanche photodiode is though integrated with the readout circuit for 3D imaging but no ideal dynamic range is available. Although some new approaches are proposed to improve such drawback, utilizing another high-dynamic 2-D image sensor for complementing the insufficiency of the 3D sensor, more extra costs are needed.